The present invention relates to a catalyst for purifying exhaust gases from an internal combustion engine. In particular, it relates to a NOx catalyst.
It is well known in the art to use catalyst compositions, including those commonly referred to as three-way conversion catalysts (xe2x80x9cTWC catalystsxe2x80x9d) to treat the exhaust gases of internal combustion engines. Such catalysts, containing precious metals like platinum, palladium, and rhodium, have been found both to successfully promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) and to promote the reduction of nitrogen oxides (NOx) in exhaust gas, provided that the engine is operated around balanced stoichiometry for combustion (xe2x80x9ccombustion stoichiometryxe2x80x9d; i.e., between about 14.7 and 14.4 air/fuel (A/F) ratio).
However, fuel economy and global carbon dioxide (CO2) emissions have made it desirable to operate engines under lean-burn conditions, where the air-to-fuel ratio is somewhat greater than combustion stoichiometry to realize a benefit in fuel economy. Diesel and lean-burn gasoline engines generally operate under highly oxidizing conditions (i.e., using much more air than is necessary to burn the fuel), typically at air/fuel ratios greater than 14.7 and generally between 19 and 35. Under these highly lean conditions, typical three-way catalysts exhibit little activity toward NOx reduction, as their reduction activity is suppressed by the presence of excess oxygen.
The control of NOx emissions from vehicles is a worldwide environmental problem. Lean-burn, high air-to-fuel ratio, and diesel engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Development of an effective and durable catalyst for controlling NOx emissions under net oxidizing conditions accordingly is critical.
Recently, copper-ion exchanged zeolite catalysts have been shown to be active for selective reduction of NOx by hydrocarbons in the presence of excess oxygen. Platinum-ion exchanged zeolite catalyst is also known to be active for NOx reduction by hydrocarbons under lean conditions. However, this catalytic activity is significant only in a narrow temperature range around the lightoff temperature of hydrocarbon oxidation. All the known lean-NOx catalysts reported in the literature tend to lose their catalytic activity for NOx reduction when the catalyst temperature reaches well above the lightoff temperature of hydrocarbon oxidation. This narrow temperature window of the lean-NOx catalysts is considered to be one of the major technical obstacles, because it makes practical application of these catalysts difficult for lean-burn gasoline or diesel engines.). As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. Catalyst deactivation is accelerated by the presence of water vapor in the stream and water vapor suppresses the NO reduction activity even at lower temperatures. Also, sulfate formation at active catalyst sites and on catalyst support materials causes deactivation. Practical lean-NOx catalysts must overcome all three problems simultaneously before they can be considered for commercial use. In the case of sulfur poisoning, some gasoline can contain up to 1200 ppm of organo-sulfur compounds. Lean-NOx catalysts promote the conversion of such compounds to SO2 and SO3 during combustion. Such SO2 will adsorb onto the precious metal sites at temperatures below 300xc2x0 C. and thereby inhibits the catalytic conversions of CO, CxHy (hydrocarbons) and NOx. At higher temperatures with an Al2O3 catalyst carrier, SO2 is converted to SO3 to form a large-volume, low-density material, Al2(SO4)3, that alters the catalyst surface area and leads to deactivation. In the prior art, the primary solution to this problem has been to use fuels with low sulfur contents.
Another alternative is to use catalysts that selectively reduce NOx in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia or urea as a co-reductant. Selective catalytic reduction is based on the reaction of NO with hydrocarbon species activated on the catalyst surface and the subsequent reduction of NOx to N2. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, some containing base metals or precious metals that provide high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NOx catalysts are too susceptible to high temperatures, water vapor and sulfur poisoning (from SOx).
Yet another viable alternative involves using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline engines as a co-reductant and is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable non-selective catalytic reduction xe2x80x9cNSCRxe2x80x9d catalysts that work with hydrocarbon co-reductant in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
A leading catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage and reduction catalysis, commonly called the xe2x80x9clean-NOx trapxe2x80x9d. The lean-NOx trap technology can involve the catalytic oxidation of NO to NO2 by catalytic metal components effective for such oxidation, such as precious metals. However, in the lean NOx trap, the formation of NO2 is followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus xe2x80x9ctrappedxe2x80x9d, i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under stoichiometrically fuel-rich combustion conditions that effect a reduction of the released NOx (nitrate) to N2.
The lean-NOx-trap technology has been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO2 are also active in converting SO2 to SO3. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SOx adsorbs more strongly on NO2 adsorption sites than NO2, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. Such presence of SO3 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Attempts with limited success to solve such a problem have encompassed the use of selective SOx adsorbents upstream of lean NOx trap adsorbents. Furthermore, catalytic oxidation of NO to NO2 is limited in its temperature range. Oxidation of NO to NO2 by a conventional Pt-based catalyst maximizes at about 250xc2x0 C. and loses its efficiency below about 100 degrees and above about 400 degrees. Thus, the search continues in the development of systems that improve lean NOx technology with respect to temperature and sulfur considerations.
Another NOx removal technique comprises a non-thermal plasma gas treatment of NO to produce NO2 which is then combined with catalytic storage reduction treatment, e.g., a lean NOx catalyst, to enhance NOx reduction in oxygen-rich vehicle engine exhausts. In lean NOx, the NO2 from the plasma treatment is adsorbed on a nitrate-forming material, such as an alkali material, and stored as a nitrate. By using a plasma, the lean NOx catalyst can be implemented with known NOx adsorbers, and the catalyst may contain less or essentially no precious metals, such as Pt, Pd and Rh, for reduction of the nitrate to N2. Accordingly, an advantage is that a method for NOx emission reduction is provided that is inexpensive and reliable. The plasma-assisted lean NOx trap can allow the life of precious metal lean NOx trap catalysts to be extended for relatively inexpensive compliance to NOx emission reduction laws. Furthermore, not only does the plasma-assisted lean NOx trap process improve the activity, durability, and temperature window of lean NOx trap catalysts, but it allows the combustion of fuels containing relatively high sulfur contents with a concomitant reduction of NOx, particularly in an oxygen-rich vehicular environment.
The exhaust of a spark ignited stoichiometric engine is relatively simple, small amounts of C6 or less hydrocarbons, mostly methane, CO and NOx. Thus xe2x80x9cthree-wayxe2x80x9d refers to the three exhaust components HC, CO and NOx. There is little particulate mass and the sulfur passes through as SO2.
By comparison a diesel exhaust is far more complex. There are great quantities of particulate mass, sulfate aerosols as sulfuric acid, larger (C20 and greater) heavy hydrocarbons, a soluble oil fraction, and large quantities of carbon monoxide, nitrous oxide, nitric oxide, nitrogen dioxide, and partially oxidized hydrocarbons of all types. Additionally supplemental post fuel injection may be used to increase reducing agent in the exhaust stream. Diesel catalysts have to be complex enough to deal with all the different species present.
A diesel catalyst formulator is faced with three basic problems. First, NOx is attracted to hydrophilic materials, but the HC reducing agent is not. Second, HC reducing agent is attracted to hydrophobic materials, but NOx is not. Third, aluminosilicates (zeolites) have pores that are so small that many of the hydrocarbons cannot be adsorbed.
What is needed in the art is a lean bum NOx exhaust gas catalyst system having improved durability, as well as effective NOx management, over extended operating time. The present invention overcomes many of the shortcomings of the prior art.
Now, according to the present invention, a NOx catalyst has been developed comprising a mixture of multiple zeolite catalyst components. The NOx catalyst may comprise a mixture of five zeolite catalyst components including two hydrophilic zeolites and three hydrophobic zeolites.
A preferred embodiment of the present invention is a NOx catalyst wherein a first component comprises a NO2 to N2 conversion catalyst component comprising a first hydrophilic zeolite with a silica to alumina ratio of about 1 to 8, with about 3 to 5 being the preferred silica to alumina ratio. The zeolite can be any type, with X type zeolite and Y type zeolite being preferred; Y type zeolite is particularly preferred. The preferred ion exchanged element is barium, preferably at a level ranging from about 12 to about 28 wt %; about 18 wt % is particularly preferred. Additionally, a stabilizing agent may be added. Preferably, zirconium and/or phosphorus oxide, preferably at a level up to about 3 wt %, may be added as a stabilizing agent.
A second preferred component includes an O3 conversion component comprising a second hydrophilic zeolite with a silica to alumina ratio of about 1 to about 8; a ratio of about 5 is preferred. The zeolite can be any type, with X type zeolite and Y type zeolite preferred; X type zeolite is particularly preferred. The preferred ion exchanged element is manganese, preferably at a level ranging from about 12 to about 31 wt %; about 18 wt % is particularly preferred. Additional stabilization with barium and/or calcium is preferred; up to about 2 wt % is particularly preferred. Further, zirconium and/or phosphorus oxide, preferably at a level up to about 3 wt %, may be added as a stabilizing agent.
A third preferred component includes a HC conversion component comprising a first hydrophobic zeolite with a silica to alumina ratio of about 25 to 80, about 25 is preferred. The zeolite can be any type, with Beta and ZSM-5 being preferred; ZSM-5 is particularly preferred. The preferred ion exchanged element is a transition metal, such as nickel, at a level ranging from about 0.1 to 2.0 wt %; at least about 1.0 wt % is preferred.
A fourth preferred component includes a N2O decomposition component comprising a second hydrophobic zeolite with a silica to alumina ratio of about 25 to about 80; a ratio of about 80 is preferred. The zeolite can be any type, with Beta and ZSM-5 being preferred; ZSM-5 is especially preferred. The preferred ion exchanged element can be copper, cobalt, rhodium, and/or palladium; the particularly preferred element is selected from rhodium, palladium, and mixtures thereof. Preferably, the ion exchanged element is utilized at a level ranging from about 0.1 to 1.0 wt %; at least about 0.2 wt % is preferred.
A fifth preferred component includes a VOC (volatile organic compounds) reducing component comprising a third hydrophobic zeolite with a silica to alumina ratio ranging from about 80 to about 280; a ratio of about 280 is preferred. The zeolite can be any type, with Beta and ZSM-5 being preferred; ZSM-5 is particularly preferred. The preferred ion exchanged element is copper, cobalt, and mixtures thereof; cobalt is particularly preferred. Preferably, the ion exchanged element is utilized at a level ranging from about 0.1 to 2.0 wt %; at least about 1.0 wt % is preferred.
Resistance to steam deactivation may be accomplished by inclusion of a large pore phosphates component. Silico-alumino-phosphate (SAPO), aluminophosphate (ALPO), and mixtures thereof are preferred. Aluminophosphate (ALPO) is particularly preferred. The phosphates can contain metal ions, such as silver, nickel, and mixtures thereof. Nickel is particularly preferred.
Further, a cleanup catalyst component may be included in about the last 20% of the catalyst near the exhaust exit (e.g., the last 2xe2x80x3 of a 11xe2x80x3 long monolith). This cleanup catalyst provides CO and HC oxidation. This component may be a zeolite with silica to alumina ratio of about 1 to about 8. A ratio of about 3 is preferred. The zeolite can be any type, with Beta and ZSM-5 being preferred; ZSM-5 is particularly preferred. The preferred ion exchanged element is platinum, palladium, and mixtures thereof. Palladium is particularly preferred.
The catalyst components may be mixed to form a mixture comprising about 50 to 75 wt % hydrophilic zeolites, and about 25 to 50 wt % hydrophobic zeolites. A preferred mixture comprises about 60 to 65% hydrophilic zeolites, about 30 to 35% hydrophobic zeolites, and about 10% or less hydrophobic phosphate zeolites. A particularly preferred mixture comprises about 60 wt % hydrophilic zeolites, about 30 wt % hydrophobic zeolites, and about 10 wt % phosphates.
Preferably, the hydrophilic zeolites comprise about 50 to about 75 wt % of the first hydrophilic zeolite and about 25 to about 50 wt % of the second hydrophilic zeolite. Particularly preferred is a mixture of about 50 wt % of the first hydrophilic zeolite and about 50 wt % of the second hydrophilic zeolite.
Preferably, the hydrophobic zeolites comprise about 25 to about 50 wt % of the first hydrophobic zeolite, about 25 to about 50 wt % of the second hydrophobic zeolite, and about 25 to about 50 wt % of the third hydrophobic zeolite . Particularly preferred is a mixture of about 33 ⅓ wt % of the first hydrophobic zeolite, about 33 ⅓ wt % of the second hydrophobic zeolite, and about 33 ⅓ wt % of the third hydrophobic zeolite.
A most preferred composition comprises about 40 wt % of the first hydrophilic zeolite, about 20 wt % of the second hydrophilic zeolite, about 10 wt % of the first hydrophobic zeolite, about 10 wt % of the second hydrophobic zeolite, about 10 wt % of the third hydrophobic zeolite, and about 10 wt % of the phosphates.
The following examples are provided to further describe the invention. The examples are intended to be illustrative in nature and are not to be construed as limiting the scope of the invention.